The present invention relates to a glass fiber which comprises a core, the matrix glass of which contains at least one heavy metal oxide and at least one rare earth compound, the core being surrounded by at least two glass claddings. Furthermore, the present invention relates to a process for producing a glass fiber according to the invention, to an optical amplifier which comprises at least one glass fiber according to the invention, and to the use of the glass fiber according to the invention.
Optical amplifiers are one of the most important key components of optical communication technology. If a purely optical telecommunications signal is transmitted in a glass fiber, it is inevitable that intrinsic signal attenuation will occur. To compensate for this attenuation, it is necessary to use highly efficient optical amplifiers which are able to amplify a signal without the optical signal having to be converted into an electronic signal and then back into an optical signal. Optical amplifiers can also increase the speed of amplification, and the deterioration in the signal/noise ratio is significantly lower on account of the elimination of the conversion into electronic signals and back.
In this context, the technical demands imposed on optical amplifiers are increasing in particular on account of the continuously rising demand for ever greater bandwidths. Currently, broadband data transmission is realized using WDM (WDM “wavelength division multiplexing”) technology. Most amplifiers of the prior art operate in the C band (approx. 1528 nm to 1560 nm) and have only a limited broadband capacity, since optical amplifiers of this type have hitherto been based on Er3+-doped SiO2 glasses. Therefore, the demand for greater bandwidths has required the development of multicomponent glasses, for example heavy metal oxide glasses (HMO glasses). As manifested by their intrinsically very high refractive index (at 1.3 μm) of n>approx. 1.85, heavy metal oxide glasses have high internal electrical fields and therefore, on account of greater Stark splitting, lead to broad-band emission from the rare earth ions. However, the high refractive index of HMO glasses also leads to new problems which have to be overcome.
Various mechanisms in optical amplifier fibers can give rise to scattered light, which can lead to a deterioration in the signal/noise ratio and should therefore be removed or avoided as fully as possible.
In amplifier fibers based on SiO2, scattered light is removed by a polymer coating applied to the glass fiber. Since absorbent polymer coatings with a refractive index of n≧1.4 are available, it is readily possible for noise which is caused by reflected signals and/or scattered light from outside the fiber to be absorbed by a polymer coating of this type on the SiO2 glass fiber.
Heavy metal oxide glasses which are suitable for use as fiber amplifiers usually have a refractive index of approximately n=1.9. Polymer coatings which have hitherto been available have always had a lower refractive index than heavy metal oxide glasses. Therefore, coating with polymers of this type for absorption of scattered light causes problems, since it is only possible to provide a polymer cladding with a lower refractive index. Any coating with a cladding made from a material with a lower refractive index then leads to strong, undesired reflection at the interface between this material and the core regions or an inner cladding.
Furthermore, in conventional SiO2 amplifier fibers, there is substantially no change in refractive index at a contact location between a standard telecommunications fiber and a glass fiber of an optical amplifier, and consequently the reflection which occurs at the transition from an SiO2 glass fiber amplifier to a standard communications glass fiber is negligible.
By contrast, the high refractive index of HMO fibers means that any contact location with a standard SiO2 telecommunications glass fiber leads to strong reflection at the interface between SiO2 standard fiber and heavy metal oxide glass fiber of the optical amplifier. Since an optical amplifier is at both outputs connected to SiO2 telecommunications glass fibers or transition fibers based on SiO2 with a high numerical aperture, there is a considerable tendency for a laser resonator with standing lightwaves to form in the optical amplifier. To prevent the latter phenomenon, it is recommended for the contact locations in relation to the glass fibers to be designed at a defined or finite angle. However, this in turn leads to considerable or significant reflection which is scattered into the cladding of the fiber. Therefore, scattered light which migrates through the cladding of the fiber is reflected back and forth and it is impossible to prevent scattered light from reaching the central core region and penetrating into the latter. This scattered light will influence the inversion of the state of the rare earth ions and leads to amplification of the noise and a drop in the signal power(s) of the amplifier.
Outer, absorbent claddings for various glass systems are known from the prior art (for example K. Itoh et al., J. Non-Cryst. Sol, 256-257, 1 (1999)).
EP 1 127 858 describes a light-amplifying glass, the matrix glass of which is doped with 0.01 to 10 mol % of Er, with the matrix glass necessarily containing 20 to 80 mol % of Bi2O3, 0.01 to 10 mol % of CeO2, and at least one of B2O3 or SiO2. However, the glass fibers described in this document are only provided with standard polymer coatings. The same is true of the glasses with a high antimony oxide content described in WO 99/51537.
JP 11274613 A describes a glass fiber comprising glasses with a high refractive index, which has two glass claddings. According to this document, 10 000 ppm of absorbent material are required. However, such high levels of absorbent material influence the properties of the glass and are therefore disadvantageous.